Automotive assembly paint shops typically use two cleaning steps to remove residual, semi-dried paint from paint application equipment. The first cleaning step is referred to as a purge cycle. During a purge cycle paint is removed from color tree valves by circulating a flush solution through the paint delivery hose to the tip of the paint application device. The second cleaning step is referred to as a cap wash cycle. During a cap wash cycle flush solution is used to remove paint from the outside of paint application spray caps, bells or any similar component, that is used to atomize paint. The flush solutions used in the purge and cap wash cycles are collected and then concentrated in some manner to reduce the volume of waste generated in the process. However, most plants, particularly automotive plants, do not have an economical and efficient process to handle the large volumes of used flush solution and the corresponding waste that is generated.
Water-based flushing solutions generally contain water, alkaline agents and blends of organic water-soluble solvents. Used flushing solutions also contain paint particles and organic compounds, e.g., polymeric resins, present in the paints. As a result, the used flush solutions cannot be discarded without some environmental containment or treatment of the organic containing waste.
U.S. Pat. No. 5,836,321 describes a process of recycling a flush solution solvent used to purge a paint supply line of a paint sprayer. The process includes collection of the water-based flush solution and paint into a collection tank. The collected solution is then directed to a working tank that is connected to an ultrafiltration unit. The ultrafiltration unit concentrates used flush solution by providing for the passage of water and water-based solvents through a membrane, while restricting the passage of pigments and resin components (organics). The resin components generally have a molecular weight from 1000 to 5000. The process is stated to recover from 50% to 65% of the water and solvents from the used flush solution. The separated water and solvents are pumped to a stabilization tank, and then back to a purge solution supply tank. The process also describes the addition of fresh flush solution to make up for the volume of flush solution not recovered in the process. The solids that remain following separation of the water are collected as a heavy slurry or sludge that is pumped out into a sludge tank for subsequent disposal.
Although the above described process begins to address the problem of managing the waste generated in large automotive coating plants, the process has many drawbacks. The process provides poor recovery rates due to a slow permeation rate through the ultrafiltration membrane, and requires relatively expensive separation equipment and high equipment maintenance costs associated with the ultrafiltration unit.
There remains the need to develop a process that can treat and recycle used flush solution generated in large automotive plants. The process should include removal of paint particles and a relatively high recovery of flush solution, preferably at minimal cost and with high through-put efficiency.